[Future] Fix invalid conversion from unsigned to ptr

Fix

emper/io/Future.cpp:184:35: error: invalid conversion from ‘uint64_t’ {aka ‘long unsigned int’} to ‘void*’ [-fpermissive] 184 | io_uring_prep_cancel(sqe, user_data, 0); | ^~~~~~~~~ | | | uint64_t {aka long unsigned int}

emper/io/Future.cpp

@@ -192,7 +192,11 @@ void CancelWrapper::prepareSqeInternal(struct io_uring_sqe* sqe) {
 		assert(future.submitter == IoContext::workerIo);
 	}
 	uint64_t user_data = IoContext::createFutureTag(future);
-	io_uring_prep_cancel(sqe, user_data, 0);
+
+	static_assert(sizeof(void*) == sizeof(std::uint64_t));
+	void* user_data_as_ptr = reinterpret_cast<void*>(user_data);
+
+	io_uring_prep_cancel(sqe, user_data_as_ptr, 0);
 }
 
 void Future::completeSynchronously() {